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Genetic recombination 4 Crossing over of linked genes- Now they DO assort independently of each other! 4 Genetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency. 4 Linkage maps Genetic map based on recombination frequencies Animate!

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Alterations in Chromosome Structure 4 Can result from non-reciprocal crossovers or weak area of chromosomes. 4 Deletion: removal of a chromosomal segment 4 Duplication: repeats a chromosomal segment (a fragment may join a homolog) 4 Inversion: segment reversal in a chromosome (reattaches in reverse) 4 Translocation: movement of a chromosomal segment to another

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Various effects of Chromosomal Alterations: -Homozygous deletions are usually lethal -Duplications & translocations usually deleted -Some reciprocal translocations can alter phenotype because of “POSITION EFFECT”: -The influence on a gene’s expression because of its location among neighboring genes.

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Genomic imprinting 4 Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. 4 “Something” happens to the gene after they arrive to form the zygote 4 May involve degrees of methylization (CH 3 groups) 4 Example:Fragile X syndrome: higher prevalence of disorder and in males phs/pages/fragile_x.htm

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Another example of GENOMIC IMPRINTING Prader-Willi / Angelman Syndrome- Both disorders are caused by a deletion on Chromosome 15 but: If the altered chromosome came from Dad: Prader-Willi- Mentally handicapped, short, obese hs/pages/praderwilli.htm If the altered chromosome came from Mom: Angelman’s Syndrome- motor/mental symptoms, jerky, uncontrolled laughter

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Most (but not all) Disorders are recessively Inherited. Defective alleles code for either a malfunctioning protein…. or no protein at all. Heterozygotes can be phenotypically normal if one of the normal copies is all that is needed. They are, of course, carriers. Most people with recessive disorders are born to “normal” parents. (carriers) Probability is ¼ that the mating of 2 carriers will produce a homozygous recessive. …and 2/3 that (s)he will be a carrier. Consanguinity: condition that results from shared ancestry Results in more homozygous recessives. (Stearns County Syndrome) REMEMBER!- not all disorders are recessive some are “late-acting” & occur after sexual maturity. Example- Huntington’s Disease